Abstract

The semiconductorZnO has gained substantial interest in the research community in part because of its large exciton binding energy which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of -type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev.142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys.6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. Lett.16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett.18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors2, 624 (1968)], in which was used as the -type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys.13, 1475 (1974)], junctions (1975) by Tsurkan et al. [Semiconductors6, 1183 (1975)], and Ohmic contacts by Brillson [J. Vac. Sci. Technol.15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity -type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of -type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of -type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, -type doping, band-gap engineering, devices, and nanostructures.

Received 02 February 2005Accepted 13 June 2005Published online 30 August 2005

Acknowledgments:

The research effort at VCU is funded by grants from AFOSR (monitored by Dr. T. Steiner and Dr. G. L. Witt), ONR (monitored by Dr. C. E. C. Wood), BMDO (monitored by C. W. Litton), and NSF (monitored by Dr. V. Hess). The authors owe a great depth of gratitude to Dr. R. J. Collins, Dr. D. C. Reynolds, Dr. C. W. Litton, Dr. Y. S. Park, and Dr. D. Langer for their contributions to the field of ZnO and many discussions and exchange of ideas and support. The authors are also thankful to B. Nemeth and J. Nause of Cermet, Inc., for collaborations on many ZnO-related topics and supply of high-quality melt-grown ZnO substrates. Naturally this manuscript would not have been possible without the contributions of all the authors contained in these pages and elsewhere. Finally, the authors would like to thank Professor Hosun Lee and Professor Martin Muñoz for reviewing the manuscript and their colleagues in the group for their support, encouragement, and interest in this work.